1. Field of the Invention
The present invention relates to the field of downhole formation fluid sample analysis in hydrocarbon producing wells. More particularly, the present invention relates to a method and apparatus for analyzing downhole fluid samples using molecularly imprinted polymer sensors (MIPS) for analyzing a formation fluid sample and determining the composition of downhole fluid samples including the percentage of filtrate contamination in a formation fluid sample.
2. Background of the Related Art
In wellbore exploration, drilling mud such as oil-based mud and synthetic-based mud types are used. The filtrates from these mud types generally invade the formation through the borehole wall to an extent, meaning that this filtrate must be removed as well as it can be removed from the formation by pumping in order to access the formation fluids after filtrate has been pumped out. Open hole sampling is an effective way to acquire representative reservoir fluids. Sample acquisition allows determination of critical information for assessing the economic value of reserves. In addition, optimal production strategies can be designed to handle these complex fluids. In open hole sampling, initially, the flow from the formation contains considerable filtrate, but as this filtrate is drained from the formation, the flow increasingly becomes richer in formation fluid. That is, the sampled flow from the formation contains a higher percentage of formation fluid as pumping continues.
It is well known that fluid being pumped from a wellbore undergoes a clean-up process in which the purity of the sample increases over time as filtrate is gradually removed from the formation and less filtrate appears in the sample. When extracting fluids from a formation, it is desirable to quantify the cleanup progress, that is, the degree of contamination from filtrate in real time. If it is known that there is too much filtrate contamination in the sample (for example, more than about 10%), then there is may be no reason to collect the formation fluid sample in a sample tank until the contamination level drops to an acceptable level. Thus, there is a need for a method and apparatus for directly analyzing a fluid sample and determining percentage of filtrate contamination in a sample.
Molecularly imprinted polymer sensors (MIPS) are now being used to analyze gases in laboratory settings at 1 atmosphere and at room temperature. U.S. Patent Application Publication No. 20030129092 by Murray, published Jul. 10, 2003, (hereinafter “Murray”), which is incorporated herein by reference in its entirety, describes a molecularly imprinted polymer solution anion sensor for measuring and detecting a wide variety of analytes.
As described in Murray, methods and apparatus for the efficient and accurate detection and quantification of analytes, including polyatomic anion analytes, are of particular interest for use in a wide range of applications. For example, such methods and apparatus are useful in the detection, monitoring, and management of environmental pollutants, including organophosphorus-based pesticides. Organophosphorus-based pesticides, including paraoxon, parathion, and diazinon are widely used in the agriculture industry. Because such materials exhibit a relatively high toxicity to many forms of plant and animal life, and also exhibit relatively high solubility in water, organophosphorus-based pesticides pose a clear threat to aquatic life and to our drinking water. Accordingly, it is imperative to be able to accurately monitor the levels of pesticides in industrial waste waters, agricultural runoffs, and other environments to determine compliance with federal and state regulations, and other safety guidelines.
Additional applications for MIPS are described in Molecularly Imprinted Polymer Sensors and Sequestering Agents, Johns Hopkins University Applied Physics Laboratory, which states that, plastics are an increasingly common part of everyday life. Most of what we consider to be plastics are organic polymers, consisting of long chains, or networks, of small carbon compounds linked together to form long heavy molecules, or macromolecules. The familiar “plastics” are typically polymers that are formed in the absence of a solvent, by a method called bulk polymerization. Bulk polymerization results in masses of entwined or networked strands to form a solid substance. The rigidity of the solid can be controlled by a process known as “crosslinking”. Crosslinking is obtained when one of the building blocks of the polymer (a monomer) has the ability to tie two or more of the strands together. The addition of crosslinking monomers forms a three dimensional network polymer that is more rigid than an uncrosslinked polymer and is insoluble in organic solvents. The greater the proportion of crosslinking monomer, the harder, or more rigid, the resulting plastic.
Polymers are common in nature and provide many of the structural molecules in living organisms. Many of the natural polymers, such as cellulose, chitin and rubber, have been employed by man to make fabrics and to use as structural materials. Some natural polymers, like rubber, are being supplanted by a large variety of synthetic polymers. An understanding of polymer structure and composition has allowed chemists to make polymers with specific desired physical properties. This is the reason why synthetic polymers have in many cases replaced other materials and natural polymers. Synthetic polymers can be made more durable and longer lasting. Their specific properties can be tailored to a purpose and so, as in the case of natural rubber, synthetic polymers can be produced that are vast improvements to their natural counterparts.
A fairly recent direction in synthetic polymer development is the introduction of molecular imprinted polymers (MIPs). These materials trace their origin back to suppositions about the operation of the human immune system by Stuart Mudd in the 30's and Linus Pauling in the 40's. Mudd's contribution was to propose the idea of complementary structures. That is to say the reason a specific antibody attacks a specific target or “antigen”, is because the shape of the antibody provides an excellent fitting cavity for the shape of the antigen. This description is very similar to the “lock and key” analogy used to explain the action of enzymes, the molecules responsible for hastening and directing biochemical reactions. In this case, the enzyme forms the lock for a particular chemical key to fit, and as this “key” is turned, the enzyme directs and hastens the production of desired products from the chemical target.
Pauling's contribution to the development of MIPs was to explain the source of the complementary shape exhibited by antibodies. He postulated how an otherwise non-specific antibody molecule could be re-organized into a specific binding molecule. He reasoned that shape specificity was obtained by using the target antigen to arrange the complementary shape of the antibody. Thus a nonspecific molecule shapes itself to the contours of a specific target and, when the target is removed, the shape is maintained to give the antibody a propensity to rebind the antigen. This process is now known as molecular imprinting or “templating”.
Molecularly imprinted polymers are made by first building a complex of a target molecule and associated attached binding molecules that possess the ability to be incorporated into a polymer. The complex is usually dissolved in a larger amount of other polymerizable molecules. The bulk of the other molecules for the polymer is made with special molecules called crosslinking monomers. These molecules have two places to bind to the polymer chain to form a rigid three dimensional structure. The crosslinkers are necessary to hold the complexing molecules in place after the target molecule or “template” is removed. It is also usual to add a solvent to the mixture. The solvent molecules get caught up in the growing polymer and leave gaps and pores in the structure to make the target complexes more accessible after the polymer is formed. Typically, after polymerization, a chunk of plastic is obtained. This chunk is ground up into a powder and the target molecule is removed by washing it out with the right solvent. The powder is left with special holes that have a memory for the target molecule are ready to recapture that specific molecule the next time it comes along.
The key step in making a MIP is to form a complex that will survive the polymerization process and leave behind a suitable set of binding sites when the target is removed. If this doesn't happen the final product won't have any memory, it's memory will be blurred and inexact and so the polymer will also bind the wrong molecules. Much of this procedure was mapped out by Professor Wulff in his early experiments. A few variations on this procedure have appeared recently directed at having surface active polymers where porosity is avoided. This is to obtain an increase in the speed of binding with a concomitant loss in capacity for binding in order to make fast responding sensors.
At present, there is no known direct methodology for accurately analyzing a downhole fluid sample or for quantifying the presence of an analyte, such as oil based mud filtrate contamination of the crude oil in samples that are collected with a wireline formation tester or an analyte ratio such as phytane-pristine ratios. Thus, there is a need for a method and apparatus for directly analyzing a sample or determining the percentage of oil based mud filtrate contamination of the crude oil in samples in a downhole environment